Radar Observations of Convective Cells and Identification of Hotspots in mountainous Terrain

COPS (Convective and Orographically-induced Precipitation Study) was an international field campaign taking place in the Upper Rhine Valley, the Black Forest and the Vosges Mountains during summer 2007. Besides a large number of ground based and airborne in situ and remote sensing instruments two radars were installed in the area. The polarimetric Doppler radar POLDIRAD of DLR was deployed at the foothills of the Vosges Mountains north of Strasbourg. A high resolution X-band radar was installed about 30 km south of POLDIRAD. Additionally two DOWs were operated in the region.

The aim of COPS was to study the orographic influence on the initiation and life cycle of convection, mainly by investigating the humidity structure. Observations with advanced instruments should help to improve the forecast skill of mesoscale numerical models, especially considering precipitation forecasts.

Among the large number of observed convective systems we will concentrate on the observations of isolated small convective rain showers which developed over the Vosges Mountains on several days. While on some days the cells were initiated at the crest line of the mountain range, on other days the cells developed on the lee side of the Vosges Mountains in the Rhine Valley. The observations by POLDIRAD and the X-band radar were enriched by high resolution simulations with the mesoscale MesoNH and COSMO-DE model.

From observations and the supporting simulations it can be deduced that different wind profiles caused the different convective initiation in relation to the Vosges Mountains. Several hotspots for convective initiation caused by orographic features favouring convergent flow were identified. On the days when weak winds were prevailing cells developed over the crest line, whereas on days when strong westerly wind were observed the initiation of convection took place in the lee of the mountains within convergent flow.